1. Field of the Invention
The present invention relates to an optical head, which controls an output of a light source according to a front monitor system, and an optical reproducing apparatus and an optical recording and reproducing apparatus mounted with the optical head.
2. Description of the Related Art
In recording and reproduction of various kinds of optical recording media, it is necessary to control an output of a light source of an optical head used for an optical recording and reproducing apparatus so as to be accurate and to be stable under changes of an environment such as temperature and also over time. In a semiconductor laser serving as a light source used for an optical head, because its output fluctuates due to fluctuation in temperature and secular change, power control is performed by Auto Power Control (APC) to realize stabilization of a power level of light beams irradiated on an optical recording medium such as an optical disk.
As a representative system of this APC, a rear monitor system and a front monitor system are known. The rear monitor system is a system for detecting a light beam, which is emitted from a rear side of a semiconductor laser chip toward an inside of a laser package, using a photo-detector provided in the laser package. In this system, it is possible to reduce the size of an optical head because the photo-detector is provided in the laser package. At present, this rear monitor system is adopted in a read-only apparatus.
However, because a light beam emitted from an end face on an opposite side of an irradiation surface of a semiconductor laser is detected in the rear monitor system, there is a problem in that detection accuracy and the like deteriorate. Therefore, the front monitor system is adopted in recording applications that require particularly high accuracy. The front monitor system is a system for separating a part of a light beam emitted from a semiconductor laser serving as a light source, detecting the separated beams to feed back the separated beams to a drive circuit for the semiconductor laser, and controlling an output of the semiconductor laser according to an intensity of the separated beams. Note that, because the front monitor system is more accurate than the rear monitor system, the front monitor system may be used for the read-only apparatus.
An optical head adopting the conventional front monitor system (e.g., Japanese Patent No. 2555239 (paragraphs 0019 to 0020 and FIG. 1)) will be explained with reference to FIGS. 12 and 13. FIG. 12 is a side view showing a schematic structure of a conventional optical head and FIG. 13 is a plan view showing the schematic structure of the conventional optical head.
As shown in FIGS. 12 and 13, a light beam emitted from a light source 1 consisting of, for example, a semiconductor laser is split into three beams for tracking error signal generation in a diffraction element 2, changed to parallel beams by a collimating lens 3, and made incident on a beam splitter 4 as P-polarized light. The beam splitter 4 has a characteristic of transmitting about 90% of the P-polarized light and reflecting about 10% of the P-polarized light.
As shown in FIG. 12, the light beams transmitted through the beam splitter 4 are reflected on a rising reflection mirror 5, whereby an optical path of the light beams is bent. Then, the light beams are changed to circularly polarized light when the light beams are transmitted through a quarter-wave plate 6 and made incident on an objective lens 7. The light beams are changed to focused light and condensed on an information track on an information recording surface of an optical recording medium 8. Note that the objective lens 7 is mounted on an actuator 9 that is movable in at least a focusing direction and a tracking direction with respect to the optical recording medium 8.
Light reflected on the information recording surface of the optical recording medium 8 is transmitted through the objective lens 7 and converted into linearly polarized light in a direction perpendicular to the forward path up to the quarter wave plate 6. Then, the linearly polarized light is reflected by the reflection mirror 5 and made incident on the beam splitter 4 as S-polarized light.
Because the beam splitter 4 reflects almost 100% of the S-polarized light, as shown in FIG. 13, the light beams reflected on the beam splitter 4 are changed to focused light in a focusing lens 10 and given astigmatism for focus error signal generation in an anamorphic lens 11. Then, the focused light is made incident on a photo-detector 12 and converted into an electric signal in a light-receiving section of the photo-detector 12.
On the other hand, as shown in FIG. 13, the light beams reflected on the beam splitter 4, which are about 10% of the light beams emitted from the light source 1, are made incident on a photo-detector 13 for front monitor. Then, the light beams are converted into an electric signal for output monitor of the light source 1 by a light-receiving section of the photo-detector 13, and the electric signal for output monitor is used for feedback control of an output of the light source 1.
In addition, there is known a method of splitting a light beam emitted from a light source and detecting a part of the light beam with a photo-detector for front monitor to thereby perform feedback control of an output of a light source (e.g., JP 2003-151167 A (paragraphs 0025 to 0033 and FIG. 1)). This conventional technique will be explained with reference to FIG. 14. As shown in the figure, a light beam emitted from the light source 1 is made incident on a beam splitter 4. The beam splitter 4 transmits almost 100% of P-polarized light and reflects almost 100% of S-polarized light. Light beams made incident as the P-polarized light are transmitted through the beam splitter 4, converted into parallel light fluxes by a collimate lens 3, and made incident on the quarter wave plate 6. The parallel light fluxes converted into circularly polarized light by the quarter wave plate 6 are guided toward the reflection mirror 5. Most of the light fluxes including those at the center are reflected by the reflection mirror 5 to travel to the objective lens 7. A part of the light fluxes in the periphery travel straight ahead without being made incident on the reflection mirror 5 and are made incident on the photo-detector 13.
The light beams traveling to the objective lens 7 form a spot on an information track of the optical recording medium 8 through the objective lens 7. Then, the light beams reflected on the optical recording medium 8 are transmitted through the objective lens 7 again, reflected on the reflection mirror 5, and made incident on the quarter wave plate 6. Then, the light beams are converted into S-polarized light by the quarter wave plate 6 and transmitted through the collimate lens 3. Thereafter, the light beams are reflected by the beam splitter 4, made incident on the photo-detector 12, and converted into an electric signal.
On the other hand, the light beams traveling to the photo-detector 13 without being made incident on the reflection mirror 5 are made incident on the light-receiving surface of the photo-detector 13 and converted into an electric signal. This electric signal is fed back to a drive circuit of the light source 1, whereby output control for the light source 1 is performed.
Further, there is known a method of reflecting or diffracting a part of peripheral light fluxes of a light beam emitted from a light source to thereby make the light fluxes incident on a photo-detector for front monitor and convert the light fluxes into an electric signal, and feeding back the electric signal to a drive circuit of the light source to thereby perform output control for the light source (e.g., JP 10-255314 A (paragraphs 0012 to 0016 and FIG. 1) and JP 2002-270940 A (paragraph 0010 and FIG. 7)). Here, the term “peripheral light fluxes” refers to light beams other than those in an effective range which are used for recording and reproduction effectively among light beams.
Moreover, there is known a method of reflecting a part of peripheral light fluxes of a light beam emitted from a light source on a part of an optical element constituting an optical head and making the light fluxes incident on a photo-detector for front monitor to perform output control for the light source. For example, a reflection surface is provided in a part of an optical element such as a diffraction grating, a collimate lens, and a beam splitter, light beams are reflected on the reflection surface, and the reflected light of the light beams is made incident on the photo-detector for front monitor.
However, in the case of the optical head described in Japanese Patent No. 2555239, a light beam from the light source is split by the beam splitter or the diffractive element into light beams for front monitor. Thus, an intensity of light beams for recording and reproduction traveling to the optical recording medium weakens. On the other hand, speedup of recording and reproduction of information is demanded, and in particular, in order to increase recording speed, it is necessary to irradiate light beams with a high intensity on the optical recording medium. Therefore, there is a problem in that it is not in accord with the demand for speedup of recording and reproduction of information to split a light beam for recording reproduction and using the split light beams as light beams for front monitor.
In the optical head described in JP 2003-151167 A, because peripheral light fluxes not used for recording and reproduction of the optical recording medium are used, an intensity of light beams for recording and reproduction does not weaken due to splitting of a light beam. However, in the photo-detector for front monitor, a portion for holding the photo-detector must be provided outside the light-receiving section, and there is a problem in that this portion blocks effective light fluxes when the photo-detector receives peripheral light fluxes. In addition, it is necessary to hold the light-receiving section of the photo-detector for front monitor substantially vertically with respect to incident light fluxes to secure stability of photoelectric conversion efficiency and a light-receiving amount. There is a problem in that the size of the optical head increases when the photo-detector for front monitor is arranged in this way.
In the optical head described in JP 10-255314 A and JP 2002-270940 A, a special optical element is required separately for guiding light beams to the photo-detector for front monitor, and there is a problem in that this leads to an increase in cost of the optical head and the optical recording and reproducing apparatus using the optical head. In addition, when light beams are diffracted using the diffractive element, if the diffraction angle is small, it is necessary to hold the photo-detector for front monitor substantially vertically with respect to incident light fluxes to secure stability of photoelectric conversion efficiency and a light-receiving amount. In this case, there is a problem in that the size of the optical head increases. If the diffraction angle is large, it is difficult to improve diffractive efficiency. In addition, there is a problem in that light diffracted in another direction changes to stray light to cause noise or adversely affect a servo system of the optical recording and reproducing apparatus.
In the optical head for reflecting a light beam on a part of the optical element, it is necessary to reflect the light beam on the optical element itself. Thus, there is a problem in that the size of the optical element itself increases. When an optical element made of glass is used, a process for providing a C-cut surface as a reflection surface is required, which leads to an increase in manufacturing cost. In addition, since the reflectance of the C-cut surface is increased by polishing the optical element, it is difficult to apply polishing treatment or mirror coating to the optical element in addition to the polishing.